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Can I modify the existing tikz coil decoration, so that the downstrokes are not drawn in a certain interval, to produce a picture like this?

coil

Edit: Previous attempts, code and MWE has been moved to or superseded by my own answer.

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17
  • I am pretty sure you can, but you should post what you have tried so far, so that those trying to help you have something to start from.
    – Peter Grill
    Oct 21, 2011 at 23:19
  • Fair enough, TikZ does have a steep learning curve. But to start, see Page 323 of the TikZ manual and get one of the coil examples to compile (I think the 2nd one is good). Figure out the appropriate \usetickzlibrary commands required and try to build a horizontal coil by replacing the (0,0)--(3,1) with (0,0)--(3,0) and eliminating the rest of the path (also eliminate the examplefill). Adjust the coil parameters to get you something close, and place a rectangle on top of it. Now you have something to start with, and people here can work on figuring out hot to hide some of the lines.
    – Peter Grill
    Oct 22, 2011 at 1:24
  • Ok, so I figured out the packages required: Besides the tikz package, you'll also need \usetikzlibrary{intersections,decorations,decorations.markings,decorations.text,decorations.pathmorphing}. This is the part of these problems that I dislike so it would be immensely helpful when asking the question so that people can focus on the hard part of the question. Oh, intersections isn't really required for the coil example but I suspect that one of the possible solutions will require that.
    – Peter Grill
    Oct 22, 2011 at 1:33
  • 1
    @Hans-PeterE.Kristiansen For the time being, just load another decoration library (for a robust solution, you would make your pm.tex file a new decoration library).
    – Andrew Stacey
    Feb 6, 2012 at 12:07
  • 1
    Ah, that's a catcode issue and is because you are merely \inputing the document. Rename your file to tikzlibrarydecorations.mydecorations.code.tex and use \tikzlibrary{decorations.mydecorations} (in addition to another decorations library as I said before).
    – Andrew Stacey
    Feb 6, 2012 at 13:20

3 Answers 3

Reset to default
34

Here is a proof-of-concept solution for drawing the coil by parametrizing it and drawing the front and back parts separately using plot paths:

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    % Define a formula for the coil.
    % This is what the numbers mean:
    % 0.3 ... how far the rings are apart
    % 0.4 ... how much from the side the rings are seen (try 0 and the same as the radius)
    % 1.5 ... radius of the rings
    \def\coil#1{
        {0.3 * (2*#1 + \t) + 0.4*sin(\t * pi r))},
        {1.5 * cos(\t * pi r)}
        }

    % Draw the part of the coil behind the rectangle
    \foreach \n in {0,1,...,10} {
        \draw[domain={0:1},smooth,variable=\t,samples=15]
            plot (\coil{\n}); 
        }

    % Draw the rectangle
    \filldraw[fill=white] (-0.5,-1) rectangle (7,1);

    % Draw the part of the coil in front of the rectangle
    \foreach \n in {0,1,...,10} {
        \draw[domain={1:2},smooth,variable=\t,samples=15,
              preaction={draw,white,line width=3pt}     % remove if undesired
             ]
            plot (\coil{\n});
        }
\end{tikzpicture}
\end{document}

result

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2
  • Thank you for the effort - it is definitely a possible solution, but I was hoping for a modification of the existing coil decoration.
    – hpekristiansen
    Oct 23, 2011 at 12:22
  • Would it be possible to add some straight ends to the coil here? Could somebody advise how?
    – user1654183
    Sep 20, 2017 at 13:08
27

Code saved in pgflibrarydecorations.mypathmorphing.code.tex

\usepgfmodule{decorations}

% coilup decoration
%
% Parameters: \pgfdecorationsegmentamplitude, \pgfdecorationsegmentlength,

\pgfdeclaredecoration{coilup}{coil}
{
  \state{coil}[switch if less than=%
    1.5\pgfdecorationsegmentlength+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude to last,
               width=+\pgfdecorationsegmentlength]
  {
    \pgfpathcurveto
    {\pgfpoint@oncoil{0    }{ 0.555}{1}}
    {\pgfpoint@oncoil{0.445}{ 1    }{2}}
    {\pgfpoint@oncoil{1    }{ 1    }{3}}
    \pgfpathmoveto{\pgfpoint@oncoil{1    }{-1    }{9}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{0.445}{-1    }{10}}
    {\pgfpoint@oncoil{0    }{-0.555}{11}}
    {\pgfpoint@oncoil{0    }{ 0    }{12}}
  }
  \state{last}[width=.5\pgfdecorationsegmentlength+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude,next state=final]
  {
    \pgfpathcurveto
    {\pgfpoint@oncoil{0    }{ 0.555}{1}}
    {\pgfpoint@oncoil{0.445}{ 1    }{2}}
    {\pgfpoint@oncoil{1    }{ 1    }{3}}
    \pgfpathmoveto{\pgfpoint@oncoil{2    }{ 0    }{6}}
  }
  \state{final}
  {
  \pgfpathmoveto{\pgfpointdecoratedpathlast}
  }
}


% coildown decoration
%
% Parameters: \pgfdecorationsegmentamplitude, \pgfdecorationsegmentlength,

\pgfdeclaredecoration{coildown}{coil}
{
  \state{coil}[switch if less than=%
    1.5\pgfdecorationsegmentlength+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude to last,
               width=+\pgfdecorationsegmentlength]
  {
    \pgfpathmoveto{\pgfpoint@oncoil{1    }{1    }{3}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{1.555}{ 1    }{4}}
    {\pgfpoint@oncoil{2    }{ 0.555}{5}}
    {\pgfpoint@oncoil{2    }{ 0    }{6}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{2    }{-0.555}{7}}
    {\pgfpoint@oncoil{1.555}{-1    }{8}}
    {\pgfpoint@oncoil{1    }{-1    }{9}}
  }
  \state{last}[width=.5\pgfdecorationsegmentlength+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude,next state=final]
  {
    \pgfpathmoveto{\pgfpoint@oncoil{1    }{ 1    }{3}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{1.555}{ 1    }{4}}
    {\pgfpoint@oncoil{2    }{ 0.555}{5}}
    {\pgfpoint@oncoil{2    }{ 0    }{6}}
  }
  \state{final}
  {
  \pgfpathlineto{\pgfpointdecoratedpathlast}
  }
}

\def\pgfpoint@oncoil#1#2#3{%
  \pgf@x=#1\pgfdecorationsegmentamplitude%
  \pgf@x=\pgfdecorationsegmentaspect\pgf@x%
  \pgf@y=#2\pgfdecorationsegmentamplitude%
  \pgf@xa=0.083333333333\pgfdecorationsegmentlength%
  \advance\pgf@x by#3\pgf@xa%
}

\endinput

and highlevel wrapper tikzlibrarydecorations.mypathmorphing.code.tex

\usetikzlibrary{decorations}
\usepgflibrary{decorations.mypathmorphing}

\endinput

Usage:

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}
\usetikzlibrary{decorations.mypathmorphing}
\begin{document}
\begin{tikzpicture}
\draw [decorate, decoration={coil,     aspect=0.4, amplitude=5 mm,  segment length=4mm}, gray, very thick] (0,0) -- (4,0);
\draw [decorate, decoration={coilup,   aspect=0.4, amplitude=5 mm,  segment length=4mm}, red] (0,0) -- (4,0);
\draw [decorate, decoration={coildown, aspect=0.4, amplitude=5 mm,  segment length=4mm}, blue] (0,0) -- (4,0);
\end{tikzpicture}
\end{document}

color coil

Result:

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{decorations.mypathmorphing}
\begin{document}
\begin{tikzpicture}
\draw [decorate, decoration={coilup, aspect=0.4, amplitude=5 mm,  segment length=4mm}] (0,0) -- (4,0);
\filldraw [fill=white] (-1,-0.4) rectangle (5,0.4);
\draw [decorate, decoration={coildown, aspect=0.4, amplitude=5 mm,  segment length=4mm}] (0,0) -- (4,0);
\end{tikzpicture}
\end{document}

coil wrap

How it was done:

@Jan Hlavacek have helped me to find the right file:

hpek@melda:/usr/local/texlive/2010/texmf-dist/tex/generic/pgf$ grep -R coil .|head -1
./libraries/decorations/pgflibrarydecorations.pathmorphing.code.tex:% coil decoration
hpek@melda:/usr/local/texlive/2010/texmf-dist/tex/generic/pgf$ cp libraries/decorations/pgflibrarydecorations.pathmorphing.code.tex ~/temp/coilMod/

and with help from @Andrew Stacey, I maneged to use the file with \usetikzlibrary{decorations.mydecorations} with the copied code in tikzlibrarydecorations.mydecorations.code.tex. The file was then modified to create the above code.

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2
  • Is it possible to remove the horizontal line at the end?
    – mas
    Mar 17, 2016 at 13:28
  • 2
    @mas: Yes. -note that this line segment is precisely the same as in the original coil decoration. You have several options to remove it: You can adjust your endpoint to be a integer multiplum of coils. You can hide the line with clip. You can edit the my code -try removing \pgfpathlineto{\pgfpointdecoratedpathlast}.
    – hpekristiansen
    Mar 17, 2016 at 21:42
9

Pgf does not have any user interface for modifying existing decorations. Your best bet will be creating a new decoration, by starting with the original coil decoration and defining two new decorations, say coil-front and coil-back.

To find the definition of the original coil decoration, cd to your pgf directory (/usr/local/texlive/2011/texmf-dist/tex/generic/pgf/ on my system, yours may be different), then run rgrep coil *, taht will find number of occurrences of the word coil, but luckily all in the same file, libraries/decorations/pgflibrarydecorations.pathmorphing.code.tex. Open that file and search for the word coil. You will then have the original definition of coil:

% coil decoration
%
% Parameters: \pgfdecorationsegmentamplitude, \pgfdecorationsegmentlength,

\pgfdeclaredecoration{coil}{coil}
{
  \state{coil}[switch if less than=%
    1.5\pgfdecorationsegmentlength+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude to last,
               width=+\pgfdecorationsegmentlength]
  {
    \pgfpathcurveto
    {\pgfpoint@oncoil{0    }{ 0.555}{1}}
    {\pgfpoint@oncoil{0.445}{ 1    }{2}}
    {\pgfpoint@oncoil{1    }{ 1    }{3}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{1.555}{ 1    }{4}}
    {\pgfpoint@oncoil{2    }{ 0.555}{5}}
    {\pgfpoint@oncoil{2    }{ 0    }{6}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{2    }{-0.555}{7}}
    {\pgfpoint@oncoil{1.555}{-1    }{8}}
    {\pgfpoint@oncoil{1    }{-1    }{9}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{0.445}{-1    }{10}}
    {\pgfpoint@oncoil{0    }{-0.555}{11}}
    {\pgfpoint@oncoil{0    }{ 0    }{12}}
  }
  \state{last}[width=.5\pgfdecorationsegmentlength+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude+%
    \pgfdecorationsegmentaspect\pgfdecorationsegmentamplitude,next state=final]
  {
    \pgfpathcurveto
    {\pgfpoint@oncoil{0    }{ 0.555}{1}}
    {\pgfpoint@oncoil{0.445}{ 1    }{2}}
    {\pgfpoint@oncoil{1    }{ 1    }{3}}
    \pgfpathcurveto
    {\pgfpoint@oncoil{1.555}{ 1    }{4}}
    {\pgfpoint@oncoil{2    }{ 0.555}{5}}
    {\pgfpoint@oncoil{2    }{ 0    }{6}}
  }
  \state{final}
  {
    \pgfpathlineto{\pgfpointdecoratedpathlast}
  }
}

\def\pgfpoint@oncoil#1#2#3{%
  \pgf@x=#1\pgfdecorationsegmentamplitude%
  \pgf@x=\pgfdecorationsegmentaspect\pgf@x%
  \pgf@y=#2\pgfdecorationsegmentamplitude%
  \pgf@xa=0.083333333333\pgfdecorationsegmentlength%
  \advance\pgf@x by#3\pgf@xa%
}

You will have to make two copies of that, and modify one to draw only the front, the other one to draw only the back. It will be an interesting exercise in pgf programming.

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1
  • hpek@melda:/usr/local/texlive/2010/texmf-dist/tex/generic/pgf$ grep -R coil . ./libraries/decorations/pgflibrarydecorations.pathmorphing.code.tex:% coil decoration
    – hpekristiansen
    Oct 23, 2011 at 19:42

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